Dave, your friend is correct at 3.4 ohms if the two parallel impedances are 6 and 8 ohms; David is correct at 2.67 ohms if the yatt, yatta, yatta, 1/4+1/8=1/X, 3/8=1/X, X=8/3, or 2.67 ohms. That's the math, but we really don't know what the actual real world result will be at any instant, since that depends on what the impedances of the two speakers would be at the frequency being played at that instant and we have no M60 impedance curve available, apparently. Having said all that, blah, blah, blah, allow)the current caused overheating to build up, again the protective circuits would cause shut down without permanent damage.

Everything that guy said is Bull$^!+ ! Math? We don't need no stink'en Math. The key is better speaker wire, that will fix all your concerns.